U.S. patent number 4,614,937 [Application Number 06/457,439] was granted by the patent office on 1986-09-30 for capacitive keyboard structure.
This patent grant is currently assigned to Commissariat a l'Energie Atomique. Invention is credited to Robert Poujois.
United States Patent |
4,614,937 |
Poujois |
September 30, 1986 |
Capacitive keyboard structure
Abstract
A capacitive keyboard structure, the keyboard comprising on one
face of a substrate a series of static sensitive keys each
associated with a pair of electrodes: an emitting electrode excited
sequentially by an alternating signal supplied by an emitting line
and a receiving electrode which is coupled capacitively to the
emitting electrode by the corresponding key and on which a
receiving line collects the variations of the signal as an effect
of possible presence of a user's finger in the neighborhood of the
key, wherein the electrodes of one same type are disposed on such
face, the electrodes of the other type being disposed on the other
face of the substrate, thus avoiding the crossing of the emitting
and receiving lines. Application to the control of an industrial or
publicly available apparatus.
Inventors: |
Poujois; Robert (Sinard,
FR) |
Assignee: |
Commissariat a l'Energie
Atomique (Paris, FR)
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Family
ID: |
9270474 |
Appl.
No.: |
06/457,439 |
Filed: |
January 12, 1983 |
Foreign Application Priority Data
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Jan 29, 1982 [FR] |
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82 01445 |
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Current U.S.
Class: |
341/33;
200/5A |
Current CPC
Class: |
H03K
17/9622 (20130101); H03K 2217/96077 (20130101) |
Current International
Class: |
H03K
17/96 (20060101); H03K 17/94 (20060101); G06F
003/02 () |
Field of
Search: |
;200/DIG.1,5A ;307/116
;340/365C,365R ;361/181,288 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2746655 |
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Oct 1977 |
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DE |
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2026745 |
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Feb 1980 |
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GB |
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1582640 |
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Jan 1981 |
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GB |
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Primary Examiner: Caldwell, Sr.; John W.
Assistant Examiner: Heim; Michael F.
Attorney, Agent or Firm: Kerkam, Stowell, Kondracki &
Clarke
Claims
What is claimed is:
1. A capacitive keyboard structure comprising a substrate having an
upper face which is positioned for access by a user and having a
lower face, a series of static sensitive keys disposed on the upper
face, each key being associated with a first and a second
electrode, one of said electrodes being an emitting electrode which
is excited sequentially by an alternating signal delivered by an
emitting line connected to said emitting electrode, and the other
of said electrodes being a receiving electrode which is coupled
capacitively to the emitting electrode by the associated static
sensitive key and which impresses on a receiving line connected to
the receiving electrode variations of the alternating signal as an
effect of the presence of a user's finger in the vicinity of such
static sensitive key, the first electrode being formed to enclose
on the upper face its associated key, and the second electrode
being disposed on the lower face below the associated key.
2. A capacitive keyboard structure according to claim 1, wherein,
the sensitive keys are disposed in parallel rows, the first
electrodes associated with the sensitive keys of each row being
electrically connected together as a group, and the second
electrodes being disposed in other parallel rows with the second
electrodes of each of said other parallel rows being electrically
connected to one another, so as to form a capacitive keyboard of
the matrix type.
3. A capacitive keyboard structure according to claim 1, wherein
the second electrode occupies a surface area on the lower face
which is of the same order of magnitude as a surface area on the
upper face occupied by the sensitive key with which it is
associated.
4. A capacitive keyboard structure according to claim 1, wherein at
least one of such faces of the substrate is provided with a
conductive casing.
5. A capacitive keyboard structure according to claim 1, wherein,
for sensing the variations in the amplitude of the alternating
signal on the receiving line, an operational amplifier which is
configured as a current integrator is connected to the receiving
line, the amplifier having a high open loop coefficient of
amplification, of the order of 10,000 to 50,000, and being looped
on itself by a feedback circuit.
6. A capacitive keyboard structure according to claim 1, wherein,
to get rid of the effect of parasitic electromagnetic signals
coming from the surroundings of the keyboard, the alternating
signal on the receiving line has substracted therefrom a signal
present on a reference line disposed in the keyboard.
7. A capacitive keyboard structure according to claim 6, wherein
the reference line is a second receiving line adjacent the
first-mentioned receiving line.
8. A capacitive keyboard structure according to claim 1, wherein an
electrically insulating layer covers at least the first
electrodes.
9. A capacitive keyboard structure according to claim 2, wherein
the first electrodes of each first-mentioned parallel row are
connected together in a ladder configuration.
10. A capacitive keyboard structure according to claim 1, wherein
said first and second electrodes and said sensitive keys comprise
layers of electrically conductive transparent materials disposed at
predetermined locations on the upper and lower faces of the
substrate.
11. A capacitive keyboard comprising a rigid substrate having an
upper face and a lower face, a plurality of static touch-sensitive
keys disposed in parallel rows on the upper face of the substrate,
each key being defined by a layer of electrically conductive
material disposed in a predetermined area on the upper face, a
first electrode for each row of keys, the first electrode
comprising a layer of electrically conductive material having first
and second portions disposed on opposite sides of the keys of the
row and extending parallel to the direction of the row, and having
a plurality of transversely extending third portions connected to
the first and second portions and disposed between each of the keys
such that each key of the row is enclosed by the first electrode, a
first signal line connected to the first electrode of each row, a
plurality of second electrodes, one for each key, disposed on the
lower face of the substrate, each second electrode comprising a
layer of electrically conductive material having a surface area of
the same order of magnitude as a surface area on the upper face
occupied by the key with which it is associated and being disposed
directly below its associated key, the second electrodes being
connected together in other parallel rows which extend
perpendicular to said first-mentioned parallel rows of keys, and
each such row of second electrodes being connected to a second
signal line, and wherein one of said first and second electrodes is
an emitting electrode which is excited sequentially by an
alternating signal delivered on its signal line and the other
electrode is a receiving electrode which is capacitively coupled to
the emitting electrode by an associated key and which receives
variations of the alternating signal as an effect of the presence
of a user's finger in the vicinity of such key.
Description
BACKGROUND OF THE INVENTION
Of course, capacitive keyboards enable a predetermined order to be
carried out by the presence of a finger in the vicinity of a
sensitive key, the expression "in the vicinity" also including
cases in which the finger is in contact with the sensitive key.
Keyboards of the kind specified are becoming more and more widely
used, both for industrial and scientific applications (for
controlling industrial or scientific apparatuses), and also in
public places and for the control of apparatuses, for example,
electric domestic appliances intended for the public.
PROBLEM
First of all, to define the various problems solved by the
invention, the known operation of a capacitive keyboard of the kind
specified will be recalled. Generally, capacitive keyboards use the
fact that the presence of a user's finger in the vicinity of one or
more conductive armatures creates electrical capacities between the
finger and the armatures, thus modifying the capacities existing
between the armatures.
Such capacitive keyboards are known which comprise, as is shown
diagrammatically in FIG. 1, sensitive keys G each associated with a
pair of subjacent electrodes--i.e., on the one hand an emitting
electrode A excited sequentially by an alternating signal supplied
by an emitting line X, and on the other hand a receiving electrode
B capacitively coupled to the emitting electrode A by the
corresponding sensitive key G. A receiving line Y collects on the
receiving electrode B the variations of the amplitude of the
alternating signal as an effect of the possible presence of the
user's finger 1 in the vicinity of the key G.
FIG. 2 is the equivalent electric circuit diagram of such a key; it
shows an emitting line (input line) X, a receiving line (output
line) Y, and two so-called "active" capacities C.sub.1, C.sub.1 ',
and a so-called "direct" capacity C.sub.2. The capacity C.sub.1
represents the capacity between the electrode A and the key G, the
capacity C.sub.1 ' represents the capacity between the key G and
the electrode B, and the capacity C.sub.2 represents the direct
capacitive coupling between the emitting electrode A and the
receiving electrode B. The presence of the user is shown
diagrammatically by a shunt 2, (FIG. 2) between the point common to
the capacities C.sub.1 and C.sub.1 ' and earth, such shunt
comprising a first capacitor 3 of a capacity close to 4 picofarads,
to represent the user's finger, and a capacity 4 of the order of 60
picofarads, to represent the capacity of the user's body in
relation to the ground. A switch I therefore show diagrammatically
the presence or absence of the user's finger 1 on the key G. The
preceding data are based on the experimental observation of the
fact that the body of a person can be represented by an
electrically conductive body which has in relation to the ground an
average capacity of about 60 picofarads, when the person is wearing
insulating footwear. When a user's finger approaches the sensitive
key G, it creates therewith a capacity which may vary from 2 to 5
picofarads, inter alia in dependence whether the finger is gloved
or not; it is the presence of that capacity which the capacitive
keyboard is intended to detect.
FIG. 2 also shows a load impedance Z situated between a receiving
line Y and earth, such impedance Z diagrammatically representing
the measuring electronics. In the known applications of such
capacitive keyboards, the two possible positions of the switch I
are detected by measuring the voltage collected at the terminals of
Z, or the current in the impedance Z, or else the dephasing between
the emitter at the input and the receiving signal at the output. In
a general way, the presence of the user's finger corresponding to a
current shunt via the line 2 is therefore expressed by a reduction
in the voltage on the line Y, due to the fact that the impedance
increases between the input X and the output Y. Clearly, therefore,
by examining the preceding variations on the impedance Z at the
output of the receiving line Y it is possible to determine in each
case that particular sensitive key G of the capacitive keyboard on
which the user's finger 1 has been place.
The description of the prior art will now be completed with a few
details about the way in which the various sensitive keys and
emitting and receiving electrodes of one and the same capacitive
keyboard connected to the outside. The simplest way in which to
design such connections is to provide an emitting line for each
emitting electrode, and a receiving line for each receiving
electrode, in which case the circuit diagram (FIG. 2) is repeated
as many times as there are sensitive keys G in the keyboard.
Nevertheless, if it is a keyboard of any size, this method of
procedure of course results in a very large number of connections,
which it is difficult to accommodate without problems in the same
keyboard structure. This is the reason why, fairly frequently, a
supply and reading are used which take a matrix form, the different
emitting and receiving electrodes being distributed at the apexes
of a rectangular matrix, as shown in FIG. 3. As shown in FIG. 3,
the different emitting electrodes A.sub.ij and receiving electrodes
B.sub.ij are distributed in a matrix network comprising inputs
X.sub.1, X.sub.2, . . . , X.sub.j, . . . for each column and output
Y.sub.1, Y.sub.2, . . . , Y.sub.i, . . . for each line of the
matrix network. FIG. 3 is limited to the electrical assembly
diagram of the emitting electrodes A.sub.ij and receiving
electrodes B.sub.ij, the different sensitive keys G.sub.ij not
being shown; they must be considered to lie outside the plane of
the drawing, above each of the pairs of electrodes A.sub.ij and
B.sub.ij. To produce and read the signals delivered by the
keyboard, a sequential alternating supply is used which arrives in
the form of pulses successively on each column X.sub.j, at the same
time supplying all the electrodes A.sub.ij corresponding to a
predetermined value of j. Then the output signal is observed by
seeking that line Y.sub.i which shows a reduction in signal. When
this observation has been made, it is known that the key G.sub.ij
was actuated, if at the same moment the column X.sub.j is excited
by the alternating train of sequential pulses. Clearly, the main
advantage of this type of matrix keyboard is the reduction in the
number of wires coming out of the keyboard to supply and read the
orders which it receives.
Nevertheless, such a matrix-type keyboard sets a problem as regards
the crossings of the interconnecting wires. This problem is solved
in some keyboards in which the corresponding emitting electrodes
A.sub.ij and receiving electrodes B.sub.ij are not exactly in the
same horizontal plane; this precisely facilitates the crossings of
the interconnecting wires. Those keyboards use two electrically
insulating layers or substrates in their construction: a first
substrate which bears the sensitive keys on its upper face, and a
second substrate which can be a printed circuit which is disposed
below the first one and bears on one face the emitting electrodes
A.sub.ij with their associated lines X.sub.j, and on the other face
the receiving electrodes B.sub.ij with their associated lines
Y.sub.i. Other keyboards are also known in which the second
substrate bears on one face the emitting electrodes A.sub.ij and
the receiving electrodes B.sub.ij and also the receiving lines
Y.sub.i, and on the other face the emitting lines X.sub.j
electrically connected to the emitting electrodes A.sub.ij by
metallized holes passing through the second substrate. These two
types of keyboard therefore have the disadvantage of requiring two
substrates in their construction, and this is expensive.
Moreover, one of the main qualities required in capacitive
keyboards lies in their sensitivity, which must be as high as
possible, at one and the same time in order to obtain an
unambiguous response each time a user's finger comes near, even
when the user has insulating gloves which considerably reduce the
capacity added by his or her finger, and also to allow the
recognition, with the least possible chance or error, of a
triggering signal, even in the presence of relatively considerable
parasitic background noise (parasitic electromagnet signals coming
from the radio, high voltage installations) and various influencing
loads which may drown out a useful signal in a considerable
environment of background noise.
Looking again at the diagram (FIG. 2), we see that the systematic
causes of the weakening of the sensitivity of detection of the
signal include the existence of the direct capacity C.sub.2 between
each pair of emitting electrodes A and receiving electrodes B,
since the intensity of the alternating signal shunted via the
capacity C.sub.2 totally escapes the influence of the user's
finger, and therefore corresponds to a portion of signal lost for
the reading of the information. The existence of the capacity
C.sub.2 is therefore another disadvantage of the capacitive
keyboards.
The present invention relates precisely to a capacitive keyboard
structure which is free from the aforementioned disadvantages,
inter alia inasmuch as it requires only one substrate, being
therefore less expensive than the constructions mentioned
hereinbefore, and enables the value of the parasitic capacity
C.sub.2 to be reduced.
BRIEF SUMMARY OF THE INVENTION
More precisely the invention relates to a capacitive keyboard
structure, of the kind comprising in known manner on that one face
of a substrate which is adjacent a user, a series of static
sensitive keys each associated with a pair of electrodes, namely on
the one hand an emitting electrode excited sequentially by an
alternating signal delivered by an emitting line, and on the other
hand a receiving electrode which is coupled capacitively to the
emitting electrode by the corresponding static sensitive key and on
which a receiving line collects the variations of the alternating
signal as an effect of the possible presence of a user's finger in
the vicinity of such static sensitive key, wherein the electrodes
of one same type are disposed on such face of the substrate, the
electrodes of the other type being disposed on the other face of
the substrate.
The term "electrodes of one same type" means the emitting
electrodes (or receiving electrodes respectively), the "electrodes
of the other type" being in that case the receiving electrodes (or
emitting electrodes).
In a first embodiment of the invention, therefore, the sensitive
keys and emitting electrodes are disposed on one face of the
substrate, in which case the receiving electrodes are disposed on
the other face; in a second embodiment the sensitive keys and
receiving electrodes are disposed on one face of the substrate, in
which case the emitting electrodes are disposed on the other face.
The invention then has the same equivalent electric circuit as that
shown in FIG. 2, repeated as many times as there are sensitive
keys. For instance, in the first embodiment there is lateral
capacitive coupling between each emitting electrode A and the
associated sensitive key G and, of course, a capacitive coupling
between each receiving electrode B and the associated sensitive key
G, which are disposed on either side of the substrate, the latter
being made, for example, of a dielectric material such as
glass.
In the structure according to the invention the problem of the
crossings of the interconnecting wires no longer arises, since the
structure can be of a matrix type or not, and the direct capacity
C.sub.2 is reduced because of the substrate thickness, which may be
stated by way of non-limitity of example to be of the order of 4 to
5 mm.
According to a particular feature of the capacitive keyboard
structure according to the invention, the electrodes of one same
type respectively enclose the sensitive keys with which they are
associated, the electrodes of the other type being disposed
respectively below the sensitive keys with which they are
associated.
In that case with the sensitive keys disposed in parallel rows, for
each row the electrodes of one same type associated with the
sensitive keys of that row are moreover all in one group, and the
electrodes of the other type are moreover disposed in other
parallel rows, in each of which the electrodes of another type are
electrically connected to one another, so as to form a capacitive
keyboard of matrix type.
According to another feature of the invention, each of the
electrodes of the other type occupies a surface area of the same
order of magnitude as the surface area of the sensitive key with
which it is associated.
According to another special feature of the invention, at least one
of such faces and other faces of the substrate is provided with a
conductive casing. This to a very large extent gets rid of any
parasitic background noise which might disturb the reading of the
state of the keyboard by reducing the signal-to-noise ratio.
However, this also produces the risk of causing parasitic
capacities electrically equivalent to shunts between the receiving
lines and earth.
According to another special feature of the invention, the
variations in the amplitude of the alternating signal coming from
each receiving line are exploited by an operational amplifier which
is mounted as a current integrator, has a high open loop
coefficient of amplification, of the order of 10,000 to 50,000, and
is looped on itself by a feedback circuit. This enables the
capacitive keyboard to be rendered insensitive to any parasitic
capacities which may exist inter alia between the receiving
electrodes or the receiving lines and earth.
The fact is that, since the coefficient of amplification A.sub.o is
very high, close to 50,000, for example, the input voltage of the
operational amplifier mounted as a current integrator is
Ve=Vs/A.sub.o (Vs being the output voltage)--i.e., such input
voltage is very low and close to zero. In other words, the input of
the current amplifier is of very low impedance; this means that in
practice the corresponding receiving line Y.sub.i is at a potential
very close to earth. Consequently, the alternating signal collected
on that line is substantially insensitive to parasitic shunt
capacities between such receiving line Y.sub.i and earth, such
capacities being thus short circuited.
According to another feature of the invention, to get rid of
parasitic electromagnetic signals coming from the surroundings of
the keyboard, the detection of the alternating signal coming from a
receiving line is performed by subtracting from such signal the
signal present on another reference line disposed in the
keyboard.
The reference line used in that case can be of any nature. For
example, it can be a line electrically independent of the keyboard
and disposed therein, possibly even connected to an independent
electrode which receives in the same manner as the emitting and
receiving electrodes of the keyboard the parasitic signals which
disturb the latter.
According to another feature of the invention the reference line is
a second receiving line adjacent the first one.
Clearly, since the parasitic signal is present on both the
receiving and the reference lines, it can be got rid of by the
subtraction of two signals.
Lastly, according to another feature of the invention, an
electrically insulating layer covers at least the electrodes of one
same type, so that they are not spoiled by users, and also for
safety reasons.
DESCRIPTION OF DRAWINGS
The invention will be better understood from the following merely
exemplary, non-limitative description of embodiment thereof, with
reference to the accompanying drawings, wherein:
FIG. 1 is a diagrammatic view of a basic element of a known
capacitive keyboard and has already been described,
FIG. 2 is an equivalent electric circuit of a basic element of a
capacitive keyboard structure and has already been described,
FIG. 3 is a diagrammatic view of a matrix-type capacitive keyboard
and has already been described,
FIG. 4 is a diagrammatic view of the preferred embodiment of the
capacitive keyboard structure according to the invention,
FIG. 5 is a diagrammatic sectional view of the preferred embodiment
of the capacitive keyboard structure according to the
invention,
FIG. 6 is a diagrammatic view of an electronic circuit enabling the
capacitive keyboard structure according to the invention to be
rendered insensitive to certain parasitic capacities, and
FIG. 7 is a diagrammatic view of the electronic circuit, so
completed that it can deliver at the output a working signal free
from any parasitic electromagnetic component which may have got
into the keyboard.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 4 shows diagrammatically a preferred embodiment of the
capacitive keyboard structure according to the invention. The
keyboard comprises an insulating substrate formed, for example, by
a glass plate 5, only a portion of which is visible, and which has
been cut away to make the drawing clearer. The glass plate 5 has an
upper face 6 on which static sensitive keys G.sub.ij in the form of
electrodes are disposed, and emitting electrodes A.sub.j and also a
lower face 7 on which receiving electrodes B.sub.ij are
disposed.
The sensitive keys G.sub.ij are disposed in parallel rows 8. For
each row 8 the emitting electrodes enclose the sensitive keys
G.sub.ij with which they are respectively associated, and they are
moreover in one single group--i.e., not individualized--so that
there corresponds to each row 8 a "emitting multi-electrode"
A.sub.j which takes the form of a ladder enclosing the sensitive
keys G.sub.ij of such row, electrically insulating spaces 9 being
left between each emitting electrode A.sub.j and the sensitive keys
G.sub.ij of the row 8 with which such electrode is associated.
Electrically insulating spaces 10 are also left between all the
emitting electrodes A.sub.j.
FIG. 4 also shows emitting lines X.sub.j supplying such alternating
signals to the emitting electrodes A.sub.j.
Each receiving electrode B.sub.ij is disposed below the sensitive
key G.sub.ij with which it is associated. The receiving electrodes
B.sub.ij are also disposed in other parallel rows 11 perpendicular
with the rows 8. In each of the other rows 11, the receiving
electrodes B.sub.ij are electrically connected to one another via
conducting lines 12 directed in accordance with the other rows 11.
The various receiving lines Y.sub.i which prolong the conducting
lines 12 are also shown.
The resulting capacitive keyboard is one of a matrix type, with
non-individualized emitting electrodes, although of course the
invention might also be put into effect by depositing on the plate
5 emitting (and receiving) electrodes which are independent of one
another, and each of which can even be associated with an emitting
(or receiving) line.
Of course a keyboard must also be imagined in which the receiving
electrodes of the other row 11 were not individualized, but reduced
to a small metallic strip.
A metallic casing 13 can be placed over the periphery of the upper
face 6, an electrically insulating space 14 being left between the
casing and the emitting electrodes A.sub.j, and electrically
insulating channels 15 also being provided through which the
emitting lines X.sub.j pass. A metal casing 13a can also be placed
on the lower face 7, electrically insulating spaces 16 being left
around the receiving electrodes B.sub.ij, and electrically
insulating channels 17 being provided through which the conductive
lines 12 pass. The casings 13 and 13a are, for instance, connected
to a fixed potential, which can be inter alia that of earth. This
more particularly gets rid of any outside parasitic influence which
might disturb the reading of the state of the keyboard.
The emitting electrodes A.sub.j can be insulated from the users by
covering the upper face 6 with an electrically insulating layer 18
which at the same time protects the sensitive keys G.sub.ij.
FIG. 5 is a sectioned diagrammatic view of the capacitive keyboard
illustrated in FIG. 4. FIG. 5 also shows the finger 1 of a user
placed adjacent a sensitive G.sub.ij.
The emitting electrodes A.sub.j and receiving electrodes B.sub.ij,
and also the sensitive keys G.sub.ij, are made, for example, from
indium and/or tin oxides, electrically conductive, transparent
materials which can be deposited on their respective faces
chemically or by a vacuum coating technique, in known manner. The
insulating layer 18 has, for example, a thickness of the order of
0.1 mm to 1 mm, and can be applied to the upper face 6 by
deposition or silk screen printing. The layer 18 can also be
transparent (an SiO.sub.2 layer, or a layer of adhesive plastic),
so that a transparent capacitive keyboard can be produced, having
application such as controlling the lighting of particular points
on a public panel disposed below the keyboard.
The total surface area of a receiving electrode B.sub.ij and of the
insulating space 16 enclosing it is, for example, equal to the
surface area of the associated sensitive key G.sub.ij ; this is
shown by dotted lines in FIG. 4. In that case, still by way of
example, rectangular or square keys G.sub.ij can be produced having
sides of 15 to 35 mm, enclosed by an insulating space 1 to 2 mm
wide, the associated receiving electrodes B.sub.ij being rectangles
or squares enclosed by an insulating space 1 to 2 mm wide. Space is
therefore better used in the capacitive keyboard structure
according to the invention than in the prior art capacitive
keyboards.
Moreover, according to the invention the active capacities C.sub.1
and C.sub.1 ' can readily be adjusted by acting on the surface of a
receiving electrode and the width of the gap separating an emitting
electrode from the associated sensitive key.
Referring again to FIG. 4, of course the arrangement of the
emitting and receiving electrodes could be reversed: the upper face
6 could receive the receiving electrodes B'.sub.i, in that case
not-individualized, associated with the receiving lines Y'.sub.i
and enclosing the sensitive keys G.sub.ij, the lower face 7 having
the emitting electrodes A'.sub.ij in this case individualized,
associated with the receiving lines X'.sub.j and disposed below the
sensitive keys G.sub.ij, the arrows in FIG. 4 corresponding to the
lines X'.sub.j and Y'.sub.i being reversed.
FIG. 6 shows diagrammatically an electronic circuit enabling the
capacitive keyboard structure according to the invention to be
rendered insensitive to certain parasitic capacities which may
exist inter alia between the receiving electrodes or the receiving
lines and earth, because of the presence of a casing in the
capacitive keyboard.
FIG. 6 shows a receiving line Y.sub.i having the reference 30 on
which an alternating signal is present of amplitude V, whose
possible variations must be detected under the influence of a
user's finger shown diagrammatically by the presence of a variable
capacity CT. A capacity C.sub.3 representing the whole of the
parasitic capacities whose effects must be suppressed is shunted
between the receiving line 30 and earth. The receiving line 30 is
connected to the negative input of an operational amplifier 31
mounted as a current integrator and having a gain A.sub.o in open
loop of the order of 10,000 to 50,000. The positive input of the
operational amplifier 31 is earthed by the connection 32, and a
feedback loop 33 comprising in parallel a capacity C and a
resistance R connects the output and the negative input of the
operational amplifier 31. The resistance R has a very high value,
of the order, for example, of 10M ohm, to define the direct input
potential of the amplifier 31. The useful signal transmitted to the
reading electronics is therefore available at the output 34 of the
amplifier 31. If we call the input and output voltages of the
amplifier Ve and Vs, we have the relationship Ve=Vs/A.sub.o, with
A.sub.o differing very little from 50,000. Consequently Ve is very
close to zero--i.e., the voltage at the terminals of the capacity
C.sub.3, which represents the whole of the parasitic capacities
which are to be got rid of, is very low, and therefore the current
flowing in C.sub.3 is negligible. The action of C.sub.3 on the
current entering the amplifier 31 is therefore practically zero. It
may also be noted that the gain of the assembly thus looped is
defined by the relationship CT/C, a formula in which CT is the
variable capacity represented by the presence of a finger adjacent
a sensitive key.
The use of an operational amplifier to get rid of parasitic
capacities is already known, with different special embodiments,
but only in the field of prior art capacitive keyboard structures.
An engineer is the art might adapt these various embodiments, of
which FIG. 6 is merely a non-limitative example, to the invention
without exceeding its scope.
FIG. 7 shows diagrammatically the electronic circuit of FIG. 6, the
circuit being modified so as to get rid of any parasitic signals
which may have got into the keyboard. FIG. 7 shows two adjacent
receiving lines Y.sub.i and Y.sub.i+1 which belong to a matrix type
reading system of the keyboard and which each supply the negative
input of an operational amplifier (i.e., 31 and 35 respectively),
each of the operational amplifiers 31 and 35 comprising a feedback
loop similar to that already described. The line Y.sub.i collects
the signals coming from the receiving electrodes B.sub.ij,
B.sub.ij+1 and B.sub.ij+2, and the receiving line Y.sub.i+1 collect
the signals coming from the receiving electrodes B.sub.i+1j,
B.sub.i+1j+1 and B.sub.i+1j+2. According to the invention the
outputs of the amplifiers 31 and 35 are connected respectively via
lines 34 and 36 to a subtractor 37 which delivers at the output 38
a signal free from all parasitic components since, if such a
component were present in the keyboard at the time of measurement,
it would be present at one and the same time on the line Y.sub.i
and on the line Y.sub.i+1, and the fact of performing the
subtraction of the two corresponding signals in the subtractor 37
therefore enables it to be eliminated.
The use of a subtractor to get rid of such parasitic signal is
known, with different special embodiments, but only in the field of
prior art capacitive keyboard structures. An engineer in the art
might adapt these various embodiments, FIG. 7 being given merely by
way of non-limitative example, to the invention without exceeding
its scope.
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